Plate loudspeakers are known in several variations of very different constructions and radiation characteristics. They only have in common that the sound radiating surface (diaphragm) is flat or only slightly bowed, i.e. it has bending radii which are much larger than the diagonal of the diaphragm. One form of plate loudspeakers is formed for example of electrostats which comprise a distributed high voltage drive, a flat metallized foil diaphragm, bulb-shaped radiation characteristics which are sharply bundled in the medium and high sound range. Another form are so-called magnetostats with a distributed electrodynamic drive, a flat metallized foil diaphragm and bulb-shaped radiation characteristics which are sharply bundled in the medium and high sound range. By contrast, absorber plates have a thin, vibration-damped, flat-laid foil diaphragm and a centrally positioned electrodynamic drive. They permit heavily damped bending wave propagation without any edge reflection, and are therefore resonance-free. So-called planars also have an electrodynamic drive, they have a flat rigid plate as the diaphragm and bulb-shaped radiation characteristics which are sharply bundled in the medium sound range. There the operating frequency range lies under the first bending vibration resonance. Finally, multiresonance plates also have an electrodynamic drive, a flat, light, bending-resistant, freely supported plate as the diaphragm. They have irregular, omnidirectional radiation characteristics, and an operating frequency range which lies under up to well above the first bending vibration resonance. Loudspeakers constructed in accordance with this principle are called multiresonance plate loudspeakers (DML=Distributed Mode Loudspeaker).
The feared bending wave resonances from cone loudspeakers should not always be expected to be harmful in plate loudspeakers. With suitable excitation and clamping techniques, material selection and plate structure, the bending vibration resonances could even form the main part of the sound event, thereby producing a new and pleasant sound experience. Such plate loudspeakers are known for example from WO 97/09842 or EP 0 924 959 A2.
The attraction of multiresonance plate loudspeakers for the user is that only a thin plate is used instead of boxes. The reproduction in the medium sound range is indisputably good. However, reproduction in the highest treble range or even in the deep ultrasonic range demanded by hi-fi audiophiles (for example 20 kHz to 50 kHz) is a problem. For that reason as yet there is no multiresonance plate loudspeaker on the market for the highest sound range.